Since fuel savings and the need to protect the environment have become a priority, it has proved necessary to produce semi-finished products, especially treads, for tires, having a reduced rolling resistance without adversely affecting other performance properties, such as wear resistance.
In addition, tire safety regulations are increasingly encouraging manufacturers to offer semi-finished products and tires that afford improved grip on roads in dry or wet conditions.
To achieve these aims of reducing the rolling resistance and the wear resistance while retaining good wet and dry grip, numerous solutions have already been tested. In particular, manufacturers have developed compositions for treads in which the carbon black was replaced by a particular precipitated silica. Indeed, some silicas have been identified as being particularly efficient for achieving these aims. These silicas have a BET specific surface area of between 100 and 250 m2/g and are termed “highly dispersible” (HD), or “highly dispersible silica” (HDS). They are used in tires with low rolling resistance, sometimes termed “green tires” in reference to the energy saving afforded to the user. As an example of such silicas, mention may especially be made of the Ultrasil 7000 silicas from Evonik, the Zeosil 1165 MP silica with a BET specific surface area approximately equal to 160 m2/g and sold by Solvay, the Zeosil 1135 MP and Zeosil 1115 MP silicas from Solvay, the Hi-Sil EZ150G silica from PPG, the Zeopol 8715, 8745 or 8755 silicas from Huber.
Another possibility for achieving these aims consists in formulating rubber compositions having large fractions by volume of reinforcing fillers and diluents, combined with synthetic elastomers.
For example, mention may be made of application WO2012/069585A1, by the Applicants, which describes compositions comprising high contents of fillers (100 to 150 phr) and a plasticizing system comprising a hydrocarbon-based resin and a plasticizing oil, the total content of plasticizers being of the order of 50 to 100 phr, with approximately 10 to 60 phr of oil and 10 to 60 phr of resin, these compositions making it possible to obtain good properties of wet grip without adversely affecting the rolling resistance. The reinforcing filler used in these compositions is a precipitated silica with a specific surface area of greater than 100 m2/g.
Application EP2336231A1 describes rubber compositions based on a diene elastomer and comprising a system of reinforcing fillers consisting of carbon black, 10 to 80 phr of a highly structured precipitated silica, and 10 to 70 phr of a relatively unstructured silica in spherical form. The highly structured porous silica has a BET specific surface area ranging from 45 to 550 m2/g and the relatively unstructured silica has a BET specific surface area ranging from 15 to 25 m2/g. The mean size of the relatively unstructured silica particles is 150 nm. These compositions, compared to a composition comprising a mixture of carbon black and a highly structured precipitated silica, have a good rolling resistance property but variable wear resistance properties.
Nonetheless, manufacturers are always seeking novel solutions to continue to improve the performance properties of compositions for tires and especially the rolling resistance, grip, wear, etc.